GNSS provides a solid foundation for research on rising sea levels

22 November 2017

For thousands of years, sea level has served as an essential, visible physical standard that affects natural and human processes. But it’s not a constant. Rapid, obvious changes in coastal water levels are caused by tides and storms. Millennial-scale, climate-induced changes are less noticeable on human timescales, but they represent a moving baseline for the local and rapid changes. Sea level changes affect marine ecosystems, habitats, shoreline and wetland erosion and accretion processes as well as human population centers.

 

To understand the local impacts of rising sea levels, researchers use direct measurements of water levels, tidal ranges and rates of change. Knowledge of modern tidal datums and their relationships to sea-level tracking ecosystems such as coral reefs, mangroves and salt marshes are essential to geologists who interpret the geo-biologic record of these environments to reconstruct paleo sea levels. The work brings together experts in coastal oceanography, geology, ecology, geodesy and marine biology.

A GNSS receiver set up on a benchmark point on the new instrument platform. Long-session GNSS observations enable researchers to establish precise control and monitor any motion of the platform.

 

Dr. Maggie Toscano, a coastal oceanographer with the Smithsonian Environmental Research Center (SERC), has long-standing research interests in sea level change. An expert in coastal and Quaternary (glacial-interglacial cycles) geology, Toscano uses geochemical dating techniques to reconstruct development of biogenic deposits that have kept pace with varying rates of sea level rise over thousands of years. “Because we tend to use biogenic recorders such as coral reef and intertidal mangrove deposits as paleo sea level gauges, it becomes very important to understand the modern tide regime in the places where we're working,” Toscano explained. “Understanding the modern ecosystem's relationship to tides and rates of sea level rise is essential to understanding the geologic record of sea level change.” 

Scientists like Toscano are worried about whether mangroves and other shallow water ecosystems can keep pace with accelerating rates of sea level rise. In the past few years, Toscano’s focus has widened to include modern sea level research as a means of documenting ongoing changes with actual measurements in the remote areas she studies.

When observing Earth’s systems, scientists seek to gather long-term measurements by going back in time as far written records allow. While tide gauges have been used for centuries to aid in navigation, physical instruments are not uniformly available. The Caribbean region has had only limited instrumentation and little to no decadal data (data acquired over ten-year periods) is available to determine rates of sea level rise and assess impacts of changing water levels on biological systems. For the past 15 years, Smithsonian was the only organization collecting tidal data in Belize and, until very recently, the only western Caribbean country north of Panama actively collecting tidal/sea level data.

Through involvement with the U.S. National Oceanographic and Atmospheric Administration (NOAA) and the Intergovernmental Oceanographic Commission’s Caribbean Tsunami Warning Network, Toscano and her Smithsonian colleagues learned how to build a modern tide station. Based on specifications for “Sentinel Sea Level Sites,” modern tide stations include geodetic control networks of benchmarks, regular GPS/GNSS surveying, stable instrument platforms and state of the art tide gauges. The researchers were awarded a Smithsonian Grand Challenges grant to build four tide stations from Maryland to Panama.

 

Global Data, Local Results
The study of sea level demonstrates how global changes can produce local effects. Dr. Philippe Hensel, a geodesist with the U.S. National Geodetic Survey (NGS) said that local or regional impacts vary widely. “We all know that global sea levels are rising,” Hensel explained, “but local effects of sea level change can be very different. Local ocean currents and vertical land motion will leave a much more nuanced signal on sea level change locally. So we're emphasizing the fact that local coastal habitats are responding to local changes in sea level. For much of the coastal research work, consistent local data is more important than developing absolute elevations tied to a national or global datum.” 

According to Hensel, the key to accurate observation of any water level is confirmed stability of the gauges and sensors used to measure water depths. Leveling between a benchmark and sensor does not reveal information about local, vertical land motion. That’s where GNSS comes in. “If you do routine GNSS measurements every year and take multiple, long static sessions, then you should be able to identify any local vertical land motion which can be compared to the sea level change data,” Hensel said.

Relating tide gauges to geodetic control networks produces benefits for human and natural communities. “When we're asking questions about the resiliency of coastal communities to sea level change we need better vertical data of the water levels,” Hensel said. “The geospatial infrastructure goes hand-in-hand with water level observations. A big part of coastal research is directed at producing reliable observations over both short and long terms.” 

Reliable data is essential at Maggie Toscano’s mangrove study sites, one of which lies on Twin Cays, Belize, about 18 km (12 mi) off the mainland coast and 3.5 km (2.2 mi) northwest of the tiny island of Carrie Bow Cay (CBC). The Cay is home to Smithsonian’s Caribbean Coral Reef Ecosystems (CCRE) Program/MarineGEO research station. Established in 1973, Carrie Bow Cay Field Station (CBC) provides housing, labs and support facilities for scientists studying the area’s coral reefs and mangrove and seagrass ecosystems. Over the years, Smithsonian’s CCRE Program has supported extensive research in coastal biology and the function of coastal habitats. CBC has produced more than 40 years of continuous data and dozens of important discoveries in marine biological research.

Set up at benchmark point CBC-South, Tim Smith uses an app on his smartphone to operate the Trimble R8s GNSS receiver. The receiver stored data internally for subsequent download and processing.

 

CBC has had an environmental monitoring system since 1999, using an instrument probe (or “sonde”) attached to a concrete dock to measure parameters such as water temperature, pH, salinity and depth. But years of exposure to seawater are taking their toll. The concrete piers are cracking and eroding and are no longer considered to be stable. “Whatever tidal data we get from the sonde, we need to factor in if the dock is moving or sinking to determine the true rate of sea level rise over time,” Toscano said. “We’re working to bring ourselves up to modern tide gauge standards and to make sure our platforms are stable.”

In 2011 Hensel established two deep-rod bench marks on the island and placed two more bench marks in the mangrove areas at Twin Cays. He used long-observation GNSS sessions on the four marks to produce baseline 3D data tied to geodetic control on the mainland. The team also used optical leveling to connect the points on CBC and tie to markers on the island’s dock and water-level gauge. In 2015 a new instrument platform was installed at CBC, roughly 75 m (250 ft) off the island’s north end. Instruments on the new tide station include a radar gauge suspended over the water; a below-sea level pressure gauge; a data logger; solar power equipment and a GOES (Geostationary Operational Environmental Satellite) transmitter. 

Geodetic surveyor Tim Smith accompanied Toscano to CBC to assist in providing elevation control on the new platform. Smith used Trimble R8s GNSS receivers to resurvey the CBC and Twin Cays bench marks as well as to establish a geodetic control point on the new platform. “The points installed in 2011 were in good condition,” he explained. “I wanted to collect long data sets to tie control to the mainland and update the earlier measurements.” Smith collected six, 24-hour static data sets while using the other R8 to collect static and fast-static data. He tied in the point on the instrument platform with 12 hours of static observations.

A skyplot on Smith’s smartphone depicts satellite status during observations. Smith used an Android app to control the receivers and monitor their performance.

 

With the geodetic control points in place, Toscano and other scientists have a solid basis for leveling to tide gauges and other instruments. They intend to make repeated measurements to the CBC control points over the coming years. The data can add confidence to the accuracy of water levels and help determine if changes in water depth are due to changes in the land or other causes. “We’re looking at everything we can to make sure that the first 15 years of sonde data reflects the correct rate of sea level change, and to account for any rates of sinking or episodic drops of the dock,” Toscano said. “We can then connect the sonde record to data from the new gauges to produce a longer trend for the region. We will monitor the new instrument platform to ensure its stability for the new tide gauges.” 

Toscano and her colleagues look forward to expanding their work to include monitoring wetland elevation changes and correlating them to measured water level changes. They also plan to make good use of the bench marks as GPS base stations from which to conduct a variety of research, including hydrographic/seismic surveying of the area, mapping field sites and taking sample site elevations. “The geodetic infrastructure and baseline GPS data we already have are major research assets at this remote location,” she said. “The scientific possibilities are very exciting.”

 

Website SERC

Website Trimble

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